1. Technical Field
The present invention relates to a lighting device and a projector, and particularly to a technology of a lighting device which uses laser beams.
2. Related Art
When laser beams as coherent lights are applied to a diffusing surface, coherent patterns called speckle noise containing randomly distributed bright points and dark points appear in some cases. The speckle noise is caused by random coherences between diffused lights at respective points on the diffusing surface. The speckle noise recognized during display of an image has undesirable effect on the image to be displayed, since the audience is dazzled by flickering light produced by the speckle noise. Technologies for reducing the speckle noise have been proposed in JP-A-7-297111 and JP-A-5-196869, for example. According to the technology disclosed in JP-A-7-297111, speckle patterns are varied by rotating a diffusion plate provided on an optical path at high speed. In this case, a particular speckle pattern cannot be easily recognized by superposing a plurality of speckle patterns. According to the technology disclosed in JP-A-5-196869, lights reciprocate between a reflection mirror and a partial reflection mirror disposed substantially in parallel with each other so that lights having passed through different optical path lengths can be emitted. In this technique, light coherence is decreased by emitting light having optical path difference corresponding to coherent length.
In case of the structure having the diffusing plate on the optical path, light loss is produced by diffusion of light on the diffusion plate. Additionally, since the relatively large-sized diffusing plate is rotated at high speed in the technology disclosed in JP-A-7-297111, power consumption and driving noise also increase. It is thus difficult to use the technology of the high-speed rotation diffusing plate particularly for private use equipment without any improvement. In case of the technology disclosed in JP-A-5-196869, light entering with inclination to the perpendicular of the mirrors reciprocates between the two mirrors while advancing obliquely. In this case, light is split while advancing obliquely between the two mirrors, and the light emission range increases as splitting of light proceeds. When the light emission range is large, the overall size of the apparatus using illumination light needs to be increased. Particularly, while the speckle noise effectively decreases with increase in the number of light divisions, the size of the structure enlarges. Various structures for emitting light having optical path difference have been proposed other than the structure disclosed in JP-A-5-196869. Examples of these structures involve a structure using fly-eye lenses of different lengths (see JP-A-60-230629), a structure using a step-type reflection mirror (JP-A-2000-199872), a structure using units of beam splitter and prism disposed in parallel (see JP-A-2001-296503), a structure using a fiber bundle, and other structures. In any of these structures, addition or enlargement of optical device is necessary as the number of light divisions increases. In this case, both the advantages of effective reduction of speckle noise and miniaturization of the structure are difficult to achieve at a time. According to the technologies in the related art, therefore, reduction of speckle noise by using a compact structure is difficult.